1. Field of the Invention
The invention relates to a circuit for the recognition of knocking of an Otto engine whrein a spark plug in the combustion chamber of a cylinder also serves as an ionic current probe. Both an excitation voltage generator and an evaluation circuit for the measuring voltage are coupled into the spark plugs high voltage ignition circuit.
Engine knocking occurs if during the combustion uncontrolled conditions of combustion occur. This knocking is detrimental to the engine and can lead to destruction of the engine. Therefore, it is important to recognize engine knocking properly and exactly.
2. Description of the Prior Art
Prior art circuits of this nature work with an direct-voltage excitation. The coupling of this direct-voltage excitation into the high-voltage circuit is carried out via a high-ohmic resistor or a high-voltage diode. These are critical constructional elements, and in case of failure of one of these, the whole ignition circuit is short-circuited so that the Otto engine ceases to operate.
It has been proposed in DE-OS 30 06 665 to obtain the direct-voltage excitation for the ionic current from the ignition current. However, in this case the excitation voltage is available just for a very short period within the operational period. This is insufficient for the detecting of a knocking condition.